trying to find the courtesy light wires in the dash (dont want to use the ignition light as it's on a timer)

i've found one wire that seems to fit the description, in that it goes to earth when the doors are opened or the interior lights are switched on, but the part that confuses me is that it's showing 12V when the interior lights are off.....

is this the right wire?

I'm sure that it is showing 12V in that state as most inferior light systems pull down to ground to activate, I.E. ur effectively seeing the 12V of the negitive side of the circuit waiting/trying to get to earth to complete/activate the inferior light system.
Best guess I have giving I can't find the fist full of pages that show me the layout for the circuits, pretty all timing nd delay (dependant on what model/make U have) is controlled by the BCM.

its in a 1994 ED XR8 Sprint

Interior light is constant 12v power, and is switched by earth wire. So the wire you have found sounds like the right one.